Piston for compressor and method of manufacturing the same

ABSTRACT

A piston for a compressor has a head portion, a neck portion, a receiving wall for receiving reactive force and a first guide wall. The receiving wall extends from a drive shaft side of the head portion towards the neck portion; is disposed on a preceding side in a rotating direction of a cam plate; and has an outer circumferential surface that slides over the inner circumferential surface of the cylinder bore. The first guide wall extends from the drive shaft side of the head portion towards the neck portion; is disposed on a following side in the rotating direction of the cam plate; and has an outer circumferential surface that slides relative to the inner circumferential surface of the cylinder bore. A recess is formed in the outer circumferential surface of the first guide wall.

BACKGROUND OF THE INVENTION

[0001] The present invention generally relates to a piston in a piston type compressor in which refrigerant gas is compressed for an air-conditioning system and a method of manufacturing the piston.

[0002] Japanese Unexamined Patent Publication No. 2000-274350 discloses a piston 110. As shown in FIG. 8A through FIG. 8C, the piston 110 includes a head portion 111 slidably inserted into a cylinder bore, a neck portion 112 engaged with a swash plate 102 of a compressor through a pair of shoes 103, and a receiving wall 113 for receiving side force F as indicated by an arrow. The receiving wall 113 extends from a lower side of the head portion 111 towards the neck portion 112 and is arranged on a predetermined side in relation to the rotating direction R of the swash plate 102.

[0003] Since the receiving wall 113 is formed on the predetermined side where the side force F is applied due to the rotating direction R of the swash plate 102, there is only a little contact area between the piston 110 and the inner circumferential surface of the cylinder bore 101 on the opposite side to the receiving wall 113. The opposite side hardly receives the side force F, thereby, the piston 110 becomes lighter. In this case, the side force F is the largest force that is generated by compression reactive force and is received by the piston 110 during a compression process.

[0004] However, since the receiving wall 113 is disposed on only one side of the piston 110 to receive the side force F due to the rotating direction of the swash plate 102, a contact area is relatively small between the piston 110 and the inner ci rcumferental surface of the cylinder bore 101. Therefore, the piston 110 reciprocates in a relatively unstable manner, and friction becomes relatively large between the piston 110 and the inner circumferential surface of the cylinder bore 101. In consequence, the compressor loses power and the friction noise becomes relatively large.

SUMMARY OF THE INVENTION

[0005] The present invention is directed to obtain a piston whose weight is relatively light and which reciprocates in a relatively stable manner and a method of manufacturing the piston.

[0006] According to the present invention, a piston is used for a compressor. The compressor has a drive shaft having a central axis and a cam plate rotatably supported by the drive shaft. The cam plate converts the rotating movement of the drive shaft into the reciprocating movement of the piston. The piston has a central axis, a head portion and a neck portion. The head portion is slidably fitted in a cylinder bore. The neck portion is connected to the head portion and is engaged with the cam plate The cam plate is rotated in a predetermined direction to define a cam plate rotating direction. The piston also has a receiving wall for receiving side force and a first guide wall. The receiving wall extends from the drive shaft side of the head portion towards the neck portion and is disposed on a preceding side in the cam plate rotating direction. The receiving wall has an outer circumferential surface that slidably contacts the inner circumferential surface of the cylinder bore. The first guide wall extends from the drive shaft side of the head portion towards the neck portion and is disposed on a following side that is opposite to the preceding side with respect to a hypothetical plane including the central axes of the drive shaft and the piston. The first guide wall has an outer circumferential surface that is continuous with the receiving wall and slides over the inner circumferential surface of the cylinder bore. A recess is formed in the outer circumferential surface of the first guide wall.

[0007] The present invention also provides a compressor. The compressor includes a housing having a cylinder bore. A drive shaft is supported by the housing and has a central axis. A cam plate is supported by the drive shaft and is rotated in a direction defined as a cam plate rotating direction. The compressor also includes a piston having a central axis. The piston has a head portion, a neck portion, a receiving wall for receiving side force and a first guide wall. The head portion is slidably fitted in a cylinder bore. The neck portion is connected to the head portion and is engaged with the cam plate for converting the rotating movement of the cam plate into the reciprocating movement of the piston. The receiving wall extends from the drive shaft side of the head portion towards the neck portion and is disposed on a preceding side in the cam plate rotating direction The receiving wall has an outer circumferential surface that slidably contacts the inner circumferential surface of the cylinder bore The first guide wall extends from the drive shaft side of the head portion towards the neck portion and is disposed on a following side that is opposite to the preceding side with respect to a hypothetical plane including the central axes of the drive shaft and the piston. The first guide wall has an outer circumferential surface that is continuous with the receiving wall and slides over the inner circumferential surface of the cylinder bore. A recess is formed in the outer circumferential surface of the first guide wall.

[0008] The present invention also provides a method of forming a piston blank that includes two piston components in use for a compressor. The compressor has a drive shaft, a cam plate and a piston. The drive shaft has a central axis. A cam plate is rotated in a predetermined direction. The piston has a central axis, a head portion, a neck portion, a receiving wall for receiving side force and a guide wall. The neck portion is connected to the head portion. The receiving wall extends from the drive shaft side of the head portion towards the neck and is disposed on a preceding side in the predetermined direction. The guide wall extends from the drive shaft side of the head portion towards the neck portion and is disposed on a following side that is opposite to the preceding side with respect to a hypothetical plane including the central axes of the drive shaft and the piston. The guide wall has an outer circumferential surface that is continuous with the receiving wall. The guide wall has a support portion connecting to the receiving wall. A recess is formed in the outer circumferential surface of the guide wall. The piston blank includes a first predetermined recess and a second predetermined recess. The method includes setting a first core in a first mold for forming the first predetermined recess such that the first core moves in a direction in which the first and second mold are separated, further setting a second core in a second mold for forming the second predetermined recess that is open to a substantially opposite side of the first predetermined recess, forming the piston blank by one of die-casting and forging, moving the first core relative to the first mold in order to break adhesion among the receiving wall, the guide wall and the first core, and separating the piston blank with the second mold from the first mold.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:

[0010]FIG. 1 is a cross-sectional view of a swash plate type variable displacement compressor of a preferred embodiment according to the present invention;

[0011]FIG. 2A is a side view of a piston of the preferred embodiment according to the present invention;

[0012]FIG. 2B is another side view opposite to FIG. 2A of the piston of the preferred embodiment according to the present invention;

[0013]FIG. 2C is a cross-sectional view taken along the line I-I in FIG. 2A of the preferred embodiment;

[0014]FIG. 3 is a partial diagram of the piston and a drive shaft of the preferred embodiment according to the present invention;

[0015]FIG. 4A is a partially enlarged cross-sectional view of a swash plate type compressor of a first alternative preferred embodiment according to the present invention;

[0016]FIG. 4B is a cross-sectional view taken along the line II-II in FIG. 4A of the first alternative embodiment;

[0017]FIG. 5A is a side view of a piston of a second alternative preferred embodiment according to the present invention;

[0018]FIG. 5B is a cross-sectional view taken along the line III-III in FIG. 5A of the second alternative embodiment;

[0019]FIG. 6 is a side view of a piston of a third alternative preferred embodiment according to the present invention;

[0020]FIG. 7A is a plane view of a piston blank and molds for casting the piston blank of the preferred embodiment according to the present invention;

[0021]FIG. 7B is a cross-sectional view taken along the line IV-IV in FIG. 7A of the preferred embodiment;

[0022]FIG. 8A is a side view of a piston according to the prior art;

[0023]FIG. 8B is another side view of the piston opposite to FIG. 8A according to the prior art; and

[0024]FIG. 8C is a cross-sectional view taken along the line V-V in FIG. 8A of the prior art piston.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0025] A preferred embodiment according to the present invention will be described. As shown in FIG. 1, a swash plate type variable displacement compressor includes a cylinder block 11 made of aluminum series metal material, a front housing 12 and a rear housing 14. In FIG. 1, the left side and the right side of the drawing respectively correspond to the front side and the rear side of the compressor. The front housing 12 is fixedly connected to the front end of the cylinder block 11. The rear housing 14 is fixedly connected to the rear end of the cylinder block 11 via a valve plate assembly 13. The cylinder block 11 and the front housing 12 define a crank chamber 15. A drive shaft 16 is rotatably placed in the crank chamber 15 An engine that is not shown in the drawing is rotatably connected to the drive shaft 16. Driving power is transmitted from the engine to the drive shaft 16, and the drive shaft 16 is rotated.

[0026] A lug plate 17 is secured to the drive shaft 16 in the crank chamber 15 so as to rotate integrally with the drive shaft 16. A swash plate 18 as a cam plate is accommodated in the crank chamber 15. The drive shaft 16 is inserted through a shaft hole 18 a that is formed at the center of the swash plate 18. The swash plate 18 is supported by the drive shaft 16 so as to slide along a central axis L of the drive shaft 16 and is inclinable with respect to the central axis L of the drive shaft 16. A hinge mechanism 19 is interposed between the lug plate 17 and the swash plate 18. The hinge mechanism 19 includes a pair of support arms 20 placed on the lug plate 17 and a pair of guide pins 21, which is secured to the swash plate 18. A spherical portion 21 a of each guide pin 21 is slidably interposed into a guide hole 20 a of each support arm 20.

[0027] Thereby, the hinge mechanism 19 between the swash plate 18 and the lug plate 17 and the support for the drive shaft 16 allow the swash plate 18 to rotate integrally with the lug plate 17 and to incline with respect to the central axis L of the drive shaft 16 as the swash plate 18 slides along the central axis L of the drive shaft 16.

[0028] A plurality of cylinder bores 22 is evenly disposed around the drive shaft 16 in the cylinder block 11 and is formed through the cylinder block 11 Only one of cylinder bores 22 is shown in the drawings. A single head piston 23 is accommodated in each of the cylinder bores 22 so as to reciprocate therein. The piston 23 and the valve plate assembly 13 respectively shut the front and rear openings of the cylinder bore 22. Thereby, a compression chamber 24 is defined in each cylinder bore 22. The volume of the compression chamber 24 varies in accordance with the reciprocating movement of the piston 23. Each of the pistons 23 is connected to the swash plate 18 through a pair of semispherical shoes 25. Thereby, the rotating movement of the swash plate 18 by the rotating drive shaft 16 is converted into the linear reciprocating movement of the piston 23 through the shoes 25.

[0029] The valve plate assembly 13 and the rear housing 14 define a suction chamber 26 and a discharge chamber 27. Each of the piston 23 travels between a top dead center and a bottom dead center. At the top dead center, the piston 23 is at the right most position in the cylinder bore 22. At the bottom dead center, the piston is the left most position in the cylinder bore 22. As each of the pistons 23 moves from the top dead center to the bottom dead center, refrigerant gas in the suction chamber 26 is introduced into the compression chamber 24 through a corresponding suction port 28 and a corresponding suction valve 29 that are formed in the valve plate assembly 13. As each of the pistons 23 moves from the bottom dead center to the top dead center, the refrigerant gas is compressed to a certain pressure and then discharged to the discharge chamber 27 through a corresponding discharge port 30 and a corresponding discharge valve 31 that are formed in the valve plate assembly 13.

[0030] A bleed passage 32, a supply passage 33 and a control valve 34 are provided in a housing of the compressor. The bleed passage 32 connects the crank chamber 15 to the suction chamber 26. The supply passage 33 connects the discharge chamber 27 to the crank chamber 15. The control valve 34 is placed on the supply passage 33.

[0031] The high pressure refrigerant gas is introduced into the crank chamber 15 from the discharge chamber 27 through the supply passage 33, and the refrigerant gas is discharged from the crank chamber 15 to the suction chamber 26 through the bleed passage 32. The above amount of the high pressure refrigerant gas and the discharged refrigerant gas is controlled by adjusting the opening degree of the control valve 34. Accordingly, the pressure in the crank chamber 15 is determined. The pressure differential between the crank chamber 15 and the compression chamber 24 varies in accordance with the pressure change in the crank chamber 15, and the inclination angle of the swash plate 18 correspondingly varies with respect to a vertical line to the central axis L of the drive shaft 16. In consequence, the stroke volume of the piston 23 changes, and the compressor displacement also varies.

[0032] For example, as the opening degree of the control valve 34 decreases, the pressure in the crank chamber 15 decreases. Therefore, the inclination angle of the swash plate 18 increases, and the stroke volume of the piston 23 increases. As a result, the displacement of the compressor increases. On the other hand, as the opening degree of the control valve 34 increases, the pressure in the crank chamber 15 increases. Therefore, the inclination angle of the swash plate 18 decreases and the displacement of the compressor is reduced.

[0033] Next, the structure of the piston 23 is described in detail. As shown in FIGS. 1, 2A through 20, the piston 23 includes a cylindrical head portion 41 and a neck portion 42 parallel to a central axis S of the piston 23. The neck portion 42 and the head portion 41 of the piston 23 respectively correspond to the front side and the rear side of the piston 23. Referring to FIG. 2C, on the front side end of the head portion 41, the distant side from the drive shaft 16 is referred to as an upper side, while the close side to the drive shaft 16 is referred to as a lower side The head portion 41 is slidably interposed into the cylinder bore 22. The neck portion 42 holds a pair of shoes 25 The piston 23 is made of aluminum series metal material and is manufactured by die-casting or forging An engaging portion 42 a is formed on the neck portion 42. A pair of shoe seats 42 b is formed in the engaging portion 42 a and receives the shoes 25 in such manner that the shoes 25 freely slides therein Film made of fluororesin such as PTFE is formed on an outer circumferential surface 41 a of the head portion 41. Thereby, the head portion 41 smoothly slides over the inner circumferential surface of the cylinder bore 22.

[0034] As shown in FIGS. 2A and 2C, a receiving wall 43 for receiving side force F extends from the lower side of the head portion 41, that is, the drive shaft side of the head portion 41, towards the neck portion 42 The receiving wall 43 is disposed on a preceding side in the rotating direction R of the swash plate 18. The rotating direction R of the swash plate 18 is defined as a swash plate rotating direction R hereafter.

[0035] As shown in FIG. 3, a diagram is viewed from a side on which the swash plate rotating direction R is clockwise. That is, when it is viewed from the side of the neck portion 42, a hypothetical plane X includes the central axis L of the drive shaft 16 and the central axis S of the piston 23. The hypothetical plane X crosses the circumferential surface 41 a of the head portion 41 of the piston 23 at intersectional points P1 and P2 The intersectional point P1 is distant from the central axis L of the drive shaft 16 and is located at 12 o'clock position On the other hand, the intersectional point P2 is close to the central axis L of the drive shaft 16 and is located at 6 o'clock position. As shown in FIGS. 2A and 2C, the outer circumferential surface 43 a of the receiving wall 43 slides over the inner circumferential surface of the cylinder bore 22 and is disposed substantially in a range of 3 to 6 o'clock. The upper side of the head portion 41 corresponds to the area around 12 o'clock position while the lower side of the head portion 41 corresponds to the area around 6 o'clock position.

[0036] The piston 23 receives reactive force from the inner circumferential surface of the cylinder bore 22. The reactive force is generated by compression reactive force and the rotating force of the swash plate 18. The reactive force is applied to a part of the outer circumferential surface 41 a of the head portion 41 after another part in accordance with the position of the piston 23. The reactive force also includes the side force F herein. The side force F is defined as the largest force of the reactive force during the compression process. The side force F is applied to the outer circumferential surface 41 a of the head portion 41 substantially in the range of 4 to 6 o'clock. Since the receiving wall 43 is continuously connected to the head portion 41, the piston 23 preferably receives some of the side force F at the receiving wall 43.

[0037] As shown in FIGS. 2B and 2C, a first guide wall 44 extends from the lower side of the head portion 41 towards the neck portion 42 The first guide wall 44 connects with the receiving wall 43. The first guide wall 44 is disposed on the opposite side of the preceding side in the swash plate rotating direction R so as to be symmetrical to a corresponding portion of the receiving wall 43 with respect to the hypothetical plane X over a longitudinal range of the first guide wall 44 The opposite side of the preceding side in the swash plate rotating direction R is defined as a following side. The outer circumferential surface 44 a of the first guide wall 44 extends within a range of 6 o'clock to 9 o'clock. Therefore, the outer circumferential surface 43 a of the receiving wall 43 and the outer circumferential surface 44 a of the first guide wall 44 are continuously arranged within the range of 3 o'clock to 9 o'clock. The receiving wall 43 and the guide wall 44 form a semicircular wall A support portion 40 extends from the middle of the head portion 41 towards the neck portion 42. The support portion is parallel to a hypothetical line Y passing through 3 o'clock and 9 o'clock when the head portion is viewed from the side of the neck portion 42 on a surface perpendicular to the central axis S of the piston 23. The support portion 40 is connected to the receiving wall 43 and the guide wall 44. The support portion 40 supports the receiving wall 43 and the guide wall 44. The receiving wall 43, the guide wall 44 and the support portion 40 form recess 45. A rib 47 connects the neck portion 42 so to the support portion 40 and the semicircular wall that includes the first guide wall 44 and the receiving wall 43.

[0038] In order to reduce the weight of the piston 23, the recess 45 is formed in the outer circumferential surface 44 a of the first guide wall 44 that slides over the inner circumferential surface of the cylinder bore 22. The contact area is triangular between the outer circumferential surface 44 a of the first guide wall 44 and the inner circumferential surface of the cylinder bore 22. The recess 45 is relatively deeply formed to minimize the thickness of the receiving wall 43. For example, if the recess 45 is not formed, a relatively large volume of the piston 23 that forms the receiving wall 43 and the first guide wall 44 is not removed. In other words, a slantingly cut semi-cylinder axially extends from the middle area of the head portion 41 to the bottom area towards the neck portion 42. The slantingly cut semi-cylinder is disposed on the front end of the head portion 41 in the range of 3 o'clock to 9 o'clock. The recess 45 is formed inside the outer circumferential surface of the slantingly cut semi-cylinder that is in contact with the inner circumferential surface of the cylinder bore 22 within the range of 6 o'clock to 9 o'clock. The slantingly cut semi-cylinder includes the receiving wall 43 for receiving the side force F, the first guide wall 44 and the support portion 40. The recess 45 is formed by a core of a mold in a process of manufacturing the piston 23 as will be mentioned later.

[0039] If the swash plate 18 rotates in the rotating direction that is opposite to the above mentioned swash plate rotating direction R, the receiving wall 43 is disposed on the following side of the preferred embodiment, and the first guide wall 44 is disposed on the preceding side of the preferred embodiment. Namely, the receiving wall 43 and the first guide wall 44 are placed in a reversed manner. However, when the above piston is manufactured, the position of the core of the mold is also shifted to form the recess 45 in the receiving wall 43 of the preferred embodiment The position of the receiving wall 43 in the above compressor corresponds to the position of the first guide wall 44 of the preferred embodiment. The first guide wall 44 is disposed on the preceding side so as to be symmetrical to the corresponding portion of the receiving wall 43 with respect to the hypothetical plane X. Therefore, in the above structure of the piston 23 of the preferred embodiment, the manufacturing process of the above two different pistons having the receiving wall 43 and the first guide wall 44 in the opposite side is facilitated by shifting the position of the core.

[0040] A second guide wall 46 extends from the head portion 41 towards the neck portion 42 on the upper side of the head portion 41. The second guide wall 46 contributes to stabilize the reciprocating movement of the piston 23. A rib 48 connects the second guide wall 46 to the neck portion 42. There is a space K between the head portion 41 and the neck portion 42. The space K is further surrounded by the receiving wall 43, the first guide wall 44, the support portion 40, the second guide wall 46, and the ribs 47, 48. An interrupting wall 51 is placed in the space K The interrupting wall 51 divides the space K into the preceding and the following sides. Although not shown in the drawings, the film made of fluororesin is coated on the outer surface of the receiving wall 43, the first guide wall 44 and the second guide wall 46 in a similar manner as the head portion 41.

[0041] In the preferred embodiment, following advantageous effects are obtained. (1) In the piston 23, the first guide wall 44 is disposed on the following side in the swash plate rotating direction R so as to be symmetrical to the corresponding portion of the receiving wall 43 with respect to the hypothetical plane X. Therefore, the reciprocating movement of the piston 23 is suitably guided by the first guide wall 44 on the following side so that the piston 23 reciprocates in a stable manner. For this reason, frictional resistance is substantially reduced between the piston 23 and the inner circumferential surface of the cylinder bore 22, and the compressor power loss and the friction noise are also substantially reduced. Since the recess 45 is formed near the outer circumferential surface 44 a of the first guide wall 44, the stable reciprocating movement of the piston 23 is highly compatible with the weight reduction of the piston 23. In consequence, especially in a swash plate type variable displacement compressor, the displacement control is stabilized due to the above compatibility.

[0042] The fluororesin film is coated on the head portion 41, the receiving wall 43, the first guide wall 44 and the second guide wall 46. Therefore, the durability of the fluororesin film is improved for the stable reciprocating movement of the piston 23.

[0043] (2) The recess 45 is formed in such manner that the contact area is frame-shaped between the outer circumferential surface 44 a of the first guide wall 44 and the inner circumferential surface of the cylinder bore 22. The support portion 40 supports the first guide wall 44. Accordingly, the strength of the first guide wall 44 is increased in the vicinity of the opening 45 a of the recess 45. The reciprocating movement of the piston 23 is suitably guided by the first guide wall 44. Therefore, as the volume of the recess 45 is increased, the stable reciprocating movement of the piston 23 is highly compatible with the weight reduction of the piston 23.

[0044] Especially in the preferred embodiment, the recess 45 is formed in such manner that the area is triangular between the outer circumferential surface 44 a of the first guide wall 44 and the inner circurnferential surface of the cylinder bore 22. Due to the triangular shape, the strength of the guide wall 44 is increased. Therefore, for example, in comparison to the case that the contact area is square-shaped between the outer circumferential surface 44 a of the first guide wall 44 and the inner circumferential surface of the cylinder bore 22, the above-mentioned strengthening effect is more effectively accomplished in the preferred embodiment.

[0045] (3) There is the space K between the head portion 41 and the neck portion 42 that is surrounded by the receiving wall 43, the first guide wall 44, the support portion 40, the second guide wall 46, and the ribs 47, 48. The space K is divided into the preceding and the following sides by the interrupting wall 51. Therefore, the interrupting wall 51 properly reinforces the connection between the head portion 41 and the neck portion 42 by supporting the receiving wall 43, the first guide wall 44, the support portion 40, the second guide wall 46 and the ribs 47, 48. As a result, the piston 23 is strengthened.

[0046] The present invention may be modified into the following alternative embodiments within the scope of the present invention. As shown in FIGS. 4A and 4B, the interrupting wall 51 is removed from the above-mentioned piston 23, and the space K is continuous between the preceding side and the following side. Therefore, the weight of the piston 23 is further reduced.

[0047] As shown in FIGS. 5A and 5B, the first guide wall 44 has a second support portion 401. The second support portion 401 is formed in the recess 45 such that two recess portions 45 are formed. The second support portion 401 reinforces the first guide wall 44. Therefore, the reduction of the piston 23 in weight is highly compatible with the stable reciprocating movement of the piston 23. The number of recesses 45 is not limited to only two as shown in FIGS. 5A and 5B, and three, four or five recess portions are formed. Through holes 49 are formed in the receiving wall 43. Thereby, lubricant in the recess 45 easily outflows from the recess 45, and the piston 23 smoothly slides over the cylinder bore 23. The through hole 49 for the outflow of the lubricant is formed in other preferred embodiments.

[0048] The rib 48 as shown in FIGS. 2A, 2B, 4A and 5A is removed from the piston 23 in an alternative embodiment. Thereby, the weight of piston 23 is further reduced in the alternative embodiment. In the pistons 23 as shown in FIGS. 2A through 2C, 5A and 5B, the interrupting wall 51 properly reinforces the piston 23. Thus, even if the rib 48 is removed form the piston 23, the strength of the piston 23 is still substantially maintained by the interrupting wall 51.

[0049] In another alternative embodiment, the recess 45 is formed in such manner that the contact area is square between the outer circumferential surface 44 a of the first guide wall 44 and the inner circumferential surface of the cylinder bore 22.

[0050] The support portion 40 is removed from the piston 23 in another alternative embodiment. Thereby, the weight of piston 23 is further reduced in the alternative embodiment.

[0051] As mentioned above, in yet another alternative embodiment, the recess 45 is formed in such manner that the contact area is frame-shaped between the outer circumferential surface 44 a of the first guide wall 44 and the inner circumferential surface of the cylinder bore 22 In the piston 23 as shown in FIGS. 2A, 2B, 4A and 5A, the outer circumferential surface 41 a of the head portion 41 and the outer circumferential surface 44 a of the first guide wall 44 around the opening of the recess 45 do not necessarily form a complete annular structure without break. The meaning of the “frame-shaped” is not limited to the above-mentioned structure without break. For example, as shown in FIG. 6, the piston 23 in which the annular structure is broken by a groove 52 is also included.

[0052] The present invention is embodied in a piston of a fixed displacement compressor The present invention is also embodied in a piston of a compressor having a wave cam plate as a cam plate.

[0053] Next, a method of manufacturing the piston according to the present invention will be described. As shown in FIGS. 7A and 7B, a method of die-casting the piston 23, which is shown in FIGS. 2A through 2C, will be described hereafter In order to obtain a single-headed piston 23, the neck portions 42 of the two pistons 23 are connected to each other in the direction of the central axis S of the piston 23, and a piston blank 50 is obtained. In this case, as shown in FIG. 7A, the recess 45 is formed symmetrically with each other in order to obtain the two pistons 23 substantially in the same shape when the piston blank is separated into two pieces. A fixed mold 53 as a first mold is connected to a movable mold 54 as a second mold at a position that corresponds to the central axis S of the piston blank 50.

[0054] A first core 55 that moves relative to the fixed mold 53 is used in order to cast the recess 45 on the side of the fixed mold 53 as a first predetermined recess. The recess 45 on the side of the movable mold 54 as a second predetermined recess is molded by a second core 55′ that is fixed to the movable mold 54. A cavity is formed to cast the piston blank 50 in the above-mentioned manner. In FIG. 7A, the reference number 56 denotes a reinforce rib of the neck portion 42, and the reference number 57 denotes a holding portion used in machining the piston blank 50 The reinforce rib 56 and the holding portion 57 are removed by cutting before the piston 23 is completed.

[0055] After pouring and cooling, the first core 55 is pulled and moved before the movable mold 54 is separated from the fixed mold 53. Thereby, the adhesiveness is removed between the first core 55 and the inner surface of the recess 45. In addition, the top of the first core 55 is separated from the recess 45, and the first core 55 can be separated from the fixed mold 53. In the above state, the adhesiveness is maintained between the molded piston blank 50 and the second core 55′ for the recess 45 at the movable mold side. When the movable mold 54 is separated from the fixed mold 53, the piston blank 50 and the second core 55′ are integrally moved. On the other hand, the first core 55 on the fixed mold side is free from the piston blank 50 In consequence, the piston blank 50 with the movable mold 54 is separated from the fixed mold 53.

[0056] According to the above method, following advantageous effects are obtained. Namely, since the piston blank 50 is cast for two pistons that are connected to each other in a row, the recesses 45 at the fixed mold and movable mold sides are symmetrical to each other across the central axis S, and the cores 55 and 55′ for forming the recess 45 are relatively deeply inserted and adhere to the recess 45 with a relatively wide area. Therefore, when the fixed mold 53 and the movable mold 54 are separated from each other, torsional force is generated since both of the cores 55 and 55′ pull the piston blank 50. However, since the first core 55 at the fixed mold side is movable, the first core 55 is separated from the recess 45 before the fixed mold 53 is separated from the piston blank 50 and the movable mold 54. Therefore, the piston blank 50 is smoothly separated from the fixed mold 53 and the movable mold 54.

[0057] The pistons 23 as shown in FIGS. 4A, 4B, 5A and 5B are also manufactured by the above-mentioned method. Especially in the piston 23 which has a plurality of the recess 45 as shown in FIGS. 5A and 5B, since the piston blank 51 contacts the first core 55 in a relatively wide area, the first core 55 strongly adheres to the piston blank 50. However, one first core 55 for forming a plurality of the recess 45 is placed in the fixed mold 53 Therefore, the recess 45 is easily released from the adhesion to the first core 55 by moving the first core 55 from the recess 45.

[0058] The present invention may be modified into the following alternative embodiments within the scope of the present invention. The first core 55 is used in the movable mold 54 while the second core 55′ is used in the fixed mold 53. In this case, the first core 55 moves relative to the movable mold 54, and the second core 55′ is fixed to the fixed mold 53.

[0059] The piston blank 50 is formed by forging. Also in this case, a first core at a fixed mold side is moved, and a piston blank 50 is relatively easily separated from the fixed mold.

[0060] Any combination of the above described preferred embodiments and or the above described alternative embodiments is practiced according to the current invention The present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein but may be modified within the scope of the appended claims. 

What is claimed is:
 1. A piston in use for a compressor, the compressor including a drive shaft having a central axis and a cam plate rotatably supported by the drive shaft, the cam plate converting rotating movement of the drive shaft into reciprocating movement of the piston, the piston having a central axis, the piston comprising: a head portion slidably fitted in a cylinder bore; a neck portion connected to the head portion, the neck portion being engaged with the cam plate, the cam plate being rotated in a direction defined as a cam plate rotating direction; a receiving wall for receiving side force extending from the drive shaft side of the head portion towards the neck portion, the receiving wall being disposed on a preceding side in the cam plate rotating direction, the receiving wall having an outer circumferential surface that slides over the inner circumferential surface of the cylinder bore; and a first guide wall extending from the drive shaft side of the head portion towards the neck portion, the first guide wall being disposed on a following side that is opposite to the preceding side with respect to a hypothetical plane including the central axes of the drive shaft and the piston, the first guide wall having an outer circumferential surface that is continuous with the receiving wall and slides over the inner circumferential surface of the cylinder bore, a recess being formed in the outer circumferential surface of the first guide wall.
 2. The piston according to claim 1, wherein the recess is formed in such manner that the outer circumferential surface of the first guide wall is frame-shaped.
 3. The piston according to claim 2, wherein the shape of the frame is a triangular shape.
 4. The piston according to claim 2, wherein the shape of the frame is a square shape.
 5. The piston according to claim 1, wherein an intersectional point between the hypothetical plane and a circumferential surface of the head portion that is further from the central axis of the cam plate is designated as 12 o'clock when the piston is viewed from a side where the cam plate rotating direction is clockwise, the outer circumferential surface of the receiving wall extending substantially in a range of 3 o'clock to 6 o'clock, the outer circumferential surface of the first guide wall extending substantially in a range of 6 o'clock to 9 o'clock.
 6. The piston according to claim 1 further comprising a rib located between the upper side of the head portion and the neck portion for connecting the head portion and the neck portion.
 7. The piston according to claim 6 further comprising an interrupting wall, wherein a space is surrounded by the head portion, the neck portion, the receiving wall, the first guide wall and the rib, the interrupting wall dividing the space into a preceding side and a following side of the cam plate rotating direction.
 8. The piston according to claim 6, wherein a space is surrounded by the head portion, the neck portion, the receiving wall, the first guide wall and the rib and is open along the cam plate rotating direction.
 9. The piston according to claim 1, wherein the first guide wall has a support portion connecting the receiving wall so as to form the recess, the support portion supporting the first guide wall.
 10. The piston according to claim 1, wherein the first guide wall has a plurality of support portions connecting the receiving wall so as to form a plurality of recesses, the support portions supporting the first guide wall, a plurality of the recesses being formed in such manner that the outer circumferential surface of the first guide wall is frame-shaped.
 11. The piston according to claim 1, wherein the compressor is a variable displacement compressor that changes a stroke volume of the piston.
 12. The piston according to claim 1, wherein a through hole is formed on the receiving wall.
 13. The piston according to claim 1, wherein a groove is formed on the outer circumferential surface of the first guide wall.
 14. The piston according to claim 1 further comprising a second guide wall extending from the upper side of the head portion towards the neck portion for guiding the piston.
 15. A piston in use for a compressor, the compressor including a drive shaft having a central axis and a cam plate rotatably supported by the drive shaft, the cam plate converting rotating movement of the drive shaft into reciprocating movement of the piston, the piston having a central axis, the piston comprising: a head portion slidably fitted in a cylinder bore; a neck portion connected to the head portion, the neck portion being engaged with the cam plate, the cam plate being rotated in a direction defined as a cam plate rotating direction; a receiving wall for receiving side force extending from the drive shaft side of the head portion towards the neck portion, the receiving wall being disposed on a preceding side in the cam plate rotating direction, the receiving wall having an outer circumferential surface that slides over the inner circumferential surface of the cylinder bore, when the piston is viewed from a side where the cam plate rotating direction is clockwise, a hypothetical plane including the central axes of the drive shaft and the piston, an intersectional point between the hypothetical plane and a circumferential surface of the head portion that is further from the central axis of the cam plate being designated as 12 o'clock, the outer circumferential surface of the receiving wall extending substantially in a range of 3 o'clock to 6 o'clock; and a first guide wall extending from the drive shaft side of the head portion towards the neck portion, the first guide wall being disposed on a following side that is opposite to the preceding side with respect to the hypothetical plane, the first guide wall having an outer circumferential surface that is continuous with the receiving wall and slides over the inner circumferential surface of the cylinder bore, the outer circumferential surface of the first guide wall extending substantially in a range of 6 o'clock to 9 o'clock, a recess being formed in the outer circumferential surface of the first guide wall, the first guide wall having a support portion connecting the receiving wall so as to form the recess, the support portion supporting the first guide wall.
 16. The piston according to claim 15, wherein the supporting portion is parallel to a hypothetical line passing through 3 o'clock and 9 o'clock.
 17. A compressor comprising: a housing having a cylinder bore, the cylinder bore having an inner circumferential surface; a drive shaft supported by the housing, the drive shaft having a central axis; a cam plate supported by the drive shaft, the cam plate being rotated in a direction defined as a cam plate rotating direction; and a piston having a central axis, the piston including; a head portion slidably fitted in the cylinder bore, a neck portion connected to the head portion, the neck portion being engaged with the cam plate for converting the rotating movement of the cam plate into reciprocating movement of the piston, a receiving wall for receiving side force extending from the drive shaft side of the head portion towards the neck portion, the receiving wall being disposed on a preceding side in the cam plate rotating direction, the receiving wall having an outer circumferential surface that slides over the inner circumferential surface of the cylinder bore, and a first guide wall extending from the drive shaft side of the head portion towards the neck portion, the first guide wall being disposed on a following side that is opposite to the preceding side with respect to a hypothetical plane including the central axes of the drive shaft and the piston, the first guide wall having an outer circumferential surface that is continuous with the receiving wall and slides over the inner circumferential surface of the cylinder bore, a recess being formed in the outer circumferential surface of the first guide wall.
 18. The compressor according to claim 17, wherein the recess is formed in such manner that the outer circumferential surface of the first guide wall is frame-shaped.
 19. The compressor according to claim 17, wherein the shape of the frame is a triangular shape.
 20. The compressor according to claim 17, wherein an intersectional point between the hypothetical plane and a circumferential surface of the head portion that is further from the central axis of the cam plate is designated as 12 o'clock when the piston is viewed from a side where the cam plate rotating direction is clockwise, the outer circumferential surface of the receiving wall extending substantially in a range of 3 o'clock to 6 o'clock, the outer circumferential surface of the guide wall extending substantially in a range of 6 o'clock to 9 o'clock.
 21. The compressor according to claim 17 further comprising a rib located between the upper side of the head portion and the neck portion for connecting the head portion and the neck portion.
 22. The compressor according to claim 21 further comprising an interrupting wall, wherein a space is surrounded by the head portion, the neck portion, the receiving wall, the first guide wall and the rib, the interrupting wall dividing the space into a preceding side and a following side of the cam plate rotating direction.
 23. The compressor according to claim 21, wherein a space is surrounded by the head portion, the neck portion, the receiving wall, the first guide wall and the rib and is open along the cam plate rotating direction.
 24. The compressor according to claim 17, wherein the first guide wall has a support portion connecting the receiving wall so as to form the recess, the support portion supporting the first guide wall.
 25. The compressor according to claim 17, wherein the first guide wall has a plurality of support portions connecting the receiving wall so as to form a plurality of recesses, the support portions supporting the first guide wall, a plurality of the recesses being formed in such manner that the outer circumferential surface of the first guide wall is frame-shaped.
 26. The compressor according to claim 17, wherein the compressor is a variable displacement compressor that changes a stroke volume of the piston.
 27. The compressor according to claim 17, wherein the piston further comprises a second guide wall extending from the upper side of the head portion towards the neck portion for guiding the piston.
 28. A method of providing a piston blank that includes two piston components in use for a compressor including a drive shaft having a central axis, a cam plate rotated in a predetermined direction and a piston having a central axis, a head portion, a neck portion connected to the head portion, a receiving wall for receiving side force extending from the drive shaft side of the head portion towards the neck portion and disposed on a preceding side in the predetermined direction, and a guide wall extending from the drive shaft side of the head portion towards the neck portion and disposed on a following side that is opposite to the preceding side with respect to a hypothetical plane including the central axes of the drive shaft and the piston, the guide wall having an outer circumferential surface that is continuous with the receiving wall, the guide wall having a support portion connecting to the receiving wall, a recess being formed in the outer circumferential surface of the guide wall, one of the piston components having a first predetermined recess, the other piston component having a second predetermined recess, the method comprising the steps of: setting a first core in a first mold for forming the first predetermined recess such that the first core moves in a direction in which the first and second molds are separated; further setting a second core in a second mold for forming the second predetermined recess that is open to a substantially opposite side of the first predetermined recess; forming the piston blank in the first and second molds by one of die-casting and forging; moving the first core relative to the first mold in order to break adhesion among the receiving wall, the guide wall and the first core; and separating the first mold from the second mold.
 29. The method according to claim 28, wherein the first mold is a fixed mold, the second mold being a movable mold. 